The present invention relates to charge coupled devices, particularly to a process for forming buried electrodes, and more particularly to a three dimensional charged coupled device using buried electrodes which produce increased spectral range and has thick substrate back illumination, large full well and dynamic range capabilities, and radiation tolerance.
Charge coupled devices (CCD) were initially developed for an electrical analog to magnetic bubble memory. See Boyle et al., "Charge Coupled Semiconductor Devices", Bell Syst. Tech. Jour. 49, 593-600 (1970). Their analog charge handling capability made them useful in applications other than digital memory storage. Since their development, CCDs have been used to build analog delay lines, transversal filters, Fourier correlators, and signal processors. However, the greatest success has been their use as solid-state image sensors.
CCDs consist of closely spaced metal-oxide-semiconductor (MOS) capacitors located on the surface of a semiconductor. With appropriate dopant concentrations and capacitor electrode voltages, a space charge region is formed within the semiconductor directly below the surface of the MOS capacitor. This space charge region generates a potential well that stores charge generated with the material. This charge is generated by a variety of sources, from thermal electrons to injection via the photoelectric effect from photons that are absorbed within the semiconductor. When the voltages of the top electrodes of the MOS capacitors are pulsed in proper sequence, the potential wells move, transporting the stored charge from one MOS capacitor to the next. In this way the CCD becomes an image sensor capable of detecting, storing, and transporting charge generated by incident photons. This powerful concept has not changed over the past decades and the architecture used to implement this concept has changed very little.
CCDs are now being used in a wide range of image sensing applications, requiring different CCD topologies. The standard front illuminated CCD, see FIG. 1A, is useful for image sensing in the visible photon energy spectrum and recently in the 1-5 keV x-ray regime. These devices find use in video camera systems and facsimile and image reproduction equipment. Unfortunately this CCD architecture cannot be used in a variety of scientific and industrial applications which require imaging in the blue, ultra-violet, and soft x-ray energy spectrum. This is due to the fact that photons within these spectra are absorbed at the top electrode layer, which is no longer transparent at these energies.
To overcome this problem, a thinned, back illuminated CCD was introduced, see FIG. 1B. See "Thinned, Back Illuminated CCDs for Short Wavelength Applications", Tektronix Tech. Note July (1991). The problem of electrode absorption is overcome by turning the CCD upside-down and illuminating the backside of the substrate material. Unfortunately, the E-field generated by the topside electrodes is not strong enough to reach through to the backside of a standard device, as shown in FIG. 1A. Therefore, the substrate material is thinned from the backside until the E-field can reach through, typically 10-20 .mu.m (see FIG. 1B). Though high quality thinned, back illuminated devices are fabricated in this manner they are expensive and extremely fragile.
Image sensing and photon counting at higher x-ray energies from 1 keV to 30 keV, is an expanding market for CCDs. See Flint, "CCD X-ray Detection", EEV Tech Note November (1991). To provide this energy range, CCDs are being developed using a thick epitaxial, deep depletion layer or region with a relatively thin substrate, see FIG. 1C. Unfortunately, there is still a problem of collecting charge deep from within the substrate, generated by the high energy x-ray photons. A 100 .mu.m to 300 .mu.m thick, high resistivity epitaxial layer is required to extend the E-field into the semiconductor. Yet such epitaxial layers are difficult to fabricate, see Flint supra, and the E-field at these depths is still too weak to overcome any lateral movement of the generated charge caused by diffusion. These devices also suffer from charge spreading at higher photon energies. The charge will drift towards the semiconductor surface at an angle dependent upon its diffusion direction. Eventually the charge will be captured by the strong E-field near the surface, though this may occur several pixels away from its point of origin. The result is a loss of resolution caused by this charge spreading phenomena.
The problems outlined above are overcome by the present invention, a three dimensional CCD (3D-CCD) capable of developing a strong E-field throughout the depth of the semiconductor by using deep (buried) parallel (bulk) electrodes in the substrate material. Using backside illumination, the 3D-CCD architecture enables a single device to image photon energies from the visible, to the ultra-violet and soft x-ray, and out to higher energy x-rays of 30 keV and beyond. In the 3D-CCD, charge is transferred from bulk electrode to bulk electrode within the body of the substrate using bulk mode operation. This mode of charge transport within a semiconductor is ideal since the charge never comes in contact with charge traps located at the surface. See White, "Charge Transport Without Traps", Solid State Imaging, Proceedings of the NATO Advanced Study Institute on Solid State Imaging, pp. 275-294, September (1975). Thus, the 3D-CCD of this invention uses the entire bulk of the semiconductor for charge generation, storage, and transfer, and thus is a vast improvement over current CCD architectures that primarily use only the surface of the semiconductor substrate.